Composite resin restorative materials were introduced to the dental profession approximately thirty years ago. Since that time, incremental improvements in the properties of these materials have made new treatment modalities possible. There have been gradual changes in the clinical failure modes of these materials, due in part to material improvements, and in part to more aggressive treatment planning. The properties of these materials rely on the strengthening and toughening of an organic matrix through the addition of silane-coupled glass of ceramic particles. Recently, fiber-reinforced restorative materials have been introduced. Although a considerable amount of data has been collected concerning the effects of matrix cure, filler permit the rational design of new composites. Manipulation of filler-matrix interphase elastic properties has been hindered by experimental difficulties. Although the fast-fracture, fatigue, and wear behavior of proprietary and experimental composites have been studied, there is a lack of a design criterion or probabilistic mechanical approach that would enable estimation of the clinical lifetimes of composites and other brittle dental restorative materials. The long-term goal of this research is to develop and test a model incorporating mechanical properties of materials, finite element techniques, and statistical methods that could be used for the design of new restorative composites. In pursuit, six specific aims are proposed: 1.) to produce and characterize a series of model composites utilizing a 60:40 BISGMA:TEGDMA resin matrix and a spherical or fibrous borosilicate glass filler; variables include volume fraction of iller, silane coupling, and polymerization kinetics; 2.) to produce a series of composites with low filler volume fractions but with exaggerated filler sizes in order to study effects of filler-matrix bonding, residual stress, and matrix plasticity on Mode I and wear crack propagation direction; 3.) to refine a microstructural finite element model in order to predict the elastic properties, strength, and toughness of composites; 4.) to construct a analytical model based on finite elements that is capable of predicting crack propagation directions in composites; 5.) to develop a statistical and fracture mechanics methodology for evaluating the reinforcing capability of spherical, random chopped fiber, and woven fiber reinforcement; 6.) to use the CARES/LIFE software package, with fracture and fatigue data as input, to predict failure probabilities of composites, viscous glass ionomers, and polyacid-modified resin composites when these are used to restore certain cavity preparations.